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DS: co ordinate

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tanyatomar
 Post subject: DS: co ordinate
 Post Posted: Mon Mar 05, 2012 2:32 pm 
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Students


Posts: 51
if ab is not = 0 and point (-a,b) and (-b,a) are in same quadrant of the xy plane , is the point (-x,y) in this same quadrant??

(1) xy>0
(2) ax>0

OA: c

i selected answer B... thinking that if ax>0 => signs of a and x are same, therefore -a and -x shud point out to same side of origin for the x coordinate of the point. However now i think that confirming xy>0 is also needed because if x and y are not of same sign then even if x lies in same side of x axis (either left or right of origin) but if y is of different sign then point cannot be in the same quadrant as (-a,b)...
please let me know if my way of thinking is correct .. or if there is any other way of looking at this problem..

Tanya
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vijay.gmat2011
 Post subject: Re: DS: co ordinate
 Post Posted: Mon Mar 05, 2012 10:11 pm 
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Students


Posts: 8
you are right. Having (-a, b) and (-b, a) => both a and b should be of same sign. If both positive, II quad else IV quad

A) xy >0 - Insufficient. Though even (-x, y) will either belong to II or IV quad, it could be opposite to where (-a,b) or (-b,a)

B) ax >0 - Insufficient. Though we know the sign of x, we cannot confirm which quadrant (-x, y) will belong to due to dependency on y

A + B - Sufficient
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RonPurewal
 Post subject: Re: DS: co ordinate
 Post Posted: Wed Mar 07, 2012 8:15 pm 
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ManhattanGMAT Staff


Posts: 8057
please search the forum for existing threads before posting!
(we may restrict the posting privileges of users who create too many new threads on problems for which threads already exist, since that just makes it harder for all users to find what they are looking for.)

here is a very long and exhaustive post on this one:
post10305.html#p10305

this thread is now locked; if you have questions, please post them on the linked thread.

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